The invention relates to a communication network for wireless communication, comprising a base station, a wireless terminal and at least one repeater which is connected between the base station and the wireless terminal. In this arrangement, the base station and the terminal are configured for transmitting and receiving a communication signal on multiple information channels. The repeater is configured for receiving, passing through and retransmitting communication signals on multiple information channels. The communication network also comprises a converting device for frequency conversion of at least one information channel passed through.
In particular, the communication network can be a network within a mobile radio network. The terminal is, for example, a mobile terminal, particularly a mobile telephone.
The repeater can be constructed, for example, as a bidirectional radiofrequency amplifier which distributes, that is to say receives, the communication signals within the communication network, amplifies them during the passing-through, filters them and transmits them again. Such a repeater is used, in particular, for providing for communication between the base station and the terminal in shading areas of a wireless network such as exist, for example, at the transition from an outdoor area into an indoor area of a building. However, the amplification function is basically not required for the communication network presented here. The repeater can be equipped for a bidirectional wireless transmission and for a bidirectional wireless reception of the communication signals. However, the repeater can also transmit and/or receive the communication signals in a wire-connected manner at least in one communication direction. In this context, the communication directions are spoken of as being in a downlink direction if communication signals are conveyed in the direction of the terminal and in an uplink direction if communication signals are conveyed in the direction of the base station.
A communication network of the type initially mentioned is known, for example, from US 2010/0284446 A1. In this paper, a frequency conversion of the communication signals is carried out between the repeaters in a multi-hop chain of repeaters in order to avoid interferences between the communication signals of a terminal and the communication signals exchanged between the repeaters. The frequency spectrum used for communication between the repeaters is arranged outside the communication band assigned for communication between base station and terminal.
A communication network of the type initially mentioned is also known from US 2009/0190508 A1. In this network, a frequency conversion is carried out in a repeater in order to be able to utilize free frequencies which are currently not occupied by the primary user and are available within a communication band assigned for primary users.
In principle, the continuously increasing demand for mobile data services leads to an increasing requirement for transmission capacity. Due to the limited bandwidth available for mobile applications, technologies have been increasingly developed in recent years which provide for a multiple utilization of this resource. It has thus been possible to utilize the spatial diversity of the transmission channel by using multi-antenna systems in conjunction with novel space/time coding methods (MIMO—Multiple Input Multiple Output). These technologies can be used particularly efficiently in highly dispersive environments as can be found, for example, in the interior of buildings or in closed cabins. It was possible to establish here that the transmission capacity can be increased approximately linearly with the number of antennas. It is of significance in this context that the spatially different transmission paths are not correlated to one another or are so as little as possible, respectively.
In supplying relatively large radio cells in the outdoor area, significant increases in the data rates can only be achieved with two antennas arranged to be cross-polar, particularly with a direct line of sight (LOS) between the communication partners. The results achieved with other configurations (NLOS—Non Line Of Sight), particularly with more than two antennas, frequently do not justify the costs for the infrastructure becoming additionally necessary. One main reason for these unsatisfactory results is based on the small angular spread and the resultant strong correlation of the communication signals of different antennas in the outdoor area.
It is the object of the invention to improve a communication network of the type initially mentioned with regard to its transmission capacity by utilizing the existing resources.
This object is achieved for a communication network of the type initially mentioned in accordance with the invention in that a test receiver is additionally comprised which is configured for determining unused frequency ranges for wireless communication, in that the test receiver is connected to the converting device of the repeater, in that the converting device is configured for carrying out the frequency conversion in at least one communication direction to a frequency from the unused frequency ranges, and in that the repeater is configured for transmitting the communication signal wirelessly with the at least one frequency-converted information channel in the at least one communication direction.
In this context, the invention is based in a first step on the approach, discussed for example in M. Muck et al., “ETSI reconfigurable radio systems: status and future directions on software defined radio and cognitive radio standards, IEEE Communications Magazine, September 2010, pp. 78-86, of a so-called cognitive radio system by means of which free unused frequency ranges are to be made accessible in the frequency spectrum suitably usable particularly for mobile applications in the outdoor area. In this context, an essential concept consists in determining the actual utilization of the available radio resources, for example with the aid of distributed sensors, and to decide centrally about their efficient utilization. For this purpose, the individual devices exchange information on the frequencies with which the information channels are transmitted in each case, the choice of the respective frequency being dependent on the available radio resources in time or in location, that is to say being carried out “cognitively”. Methods which determine the local utilization of the available frequency spectrum by measurements are presented, for example, in U.S. Pat. No. 6,882,851 B2 or U.S. Pat. No. 7,146,176 B2. The use of a relevant central database is proposed in U.S. Pat. No. 7,200,404 B2.
In a second step, the invention is based on the consideration that with respect to the frequency range suitably usable for mobile applications in the outdoor area, between 400 MHz (below that, the antenna structures needed become too large) and 3000 MHz (above that, the propagation attenuation becomes too great), only a small proportion is actually available for mobile radio. The remaining ranges are utilized either only regionally, e.g. terrestrial television, only locally, e.g. in-house WLAN, or limited in time, e.g. radar, and thus very inefficiently.
In a third step, the invention is finally based on the further consideration of utilizing the radio-frequency ranges, which, in principle, are usable for wireless communication, for the exchange of the communication signals within a wireless communication network in order to improve its transmission capacity. In other words, multiple information channels are transmitted by utilizing free radio frequencies, so-called “white spaces”, cognitively distributed to various frequencies so that the transmission capacity can be increased intelligently. In particular, this procedure makes it possible to use frequencies outside the frequency bands assigned for mobile radio so that additional resources can be included. In particular, it is not necessary to use frequencies within the frequency bands assigned for mobile radio. In this manner, the capacity limit of a communication network can be overcome which is provided by an only limited possibility of utilizing the “fully” occupied communication bands of the mobile communication.
In other words, the invention makes it possible, particularly for communication signals which are already transmitted on multiple information channels, to use resources outside the frequency band assigned in each case for wireless communication. If, for example, the MIMO technology is used for increasing the transmission capacity in an outdoor area, that is to say the communication signal is transmitted by means of spatially multiple information channels and a number of antennas, the repeater used for the communication network specified here can be configured for cognitively converting the equal-frequency MIMO signals to different frequencies, and wirelessly transmitting them, by utilizing free frequencies. In this context, the signal coding used for MIMO signals is retained so that the signals, after a corresponding frequency reconversion, can be transmitted again as equal-frequency MIMO signals via a number of antennas. Due to the fact that the MIMO signals are transmitted frequency-converted by utilizing free frequencies via the air interface, however, the unwanted restriction of the transmission capacity due to correlation of the MIMO signals transmitted in parallel is not applicable in the outdoor area.
In particular, the invention is also suitable for wirelessly transmitting MIMO signals wirelessly frequency-converted in the outdoor area by utilizing free frequency bands and for transmitting them again as MIMO signals after frequency reconversion via various antennas in an indoor area in which the advantages of the MIMO technology can be fully exploited.
The use of the invention is associated with further advantages for feeding MIMO signals into a mobile umbrella cell such as, for example, the internal space of a travelling train. With such a technology, a further impairment of the data throughput can be expected (apart from the unwanted restriction by correlation of the MIMO signals in the outdoor area). This is because it has been found that the frequency offset increasing with increasing speed of the mobile umbrella cell additionally impairs the desired decorrelation of the equal-frequency MIMO signals due to Doppler shift. Presently known methods for correcting this effect only allow a modulation rate of the mobile system.
If, in particular, a MIMO-capable repeater is used for coupling into the umbrella cell which receives MIMO signals cognitively converted in the downlink direction to various frequencies, frequency-converts these or at least one of these MIMO signals again and radiates them as equal-frequency MIMO signals into the internal space, the disadvantages associated with the Doppler effect when utilizing the MIMO technology are overcome.
The presently used term of a converting device combines those components of a corresponding assembly which are necessary for a frequency conversion of a communication signal passing through. In particular, this also comprises a frequency mixer which is occasionally described separately to a converting device.
In a preferred embodiment, the base station comprises a further converting device for frequency conversion which is connected to the test receiver and is configured for carrying out the frequency conversion in the downlink direction to a frequency from the unused frequency ranges, the converting device of the at least one repeater being configured for reconverting, particularly resetting, the frequency of the frequency-converted information channel in the downlink direction, and carrying out the frequency conversion of the at least one information channel into the frequency from the unused frequency ranges in the uplink direction.
In the aforementioned case, the base station, for example, is constructed as a cognitive base station which already exchanges communication signals at different frequencies to the said free frequencies for increasing the transmission capacity. To be able to process these frequencies, converted cognitively spatially, temporally or locally variably, of the various information channels, a cognitive terminal is basically necessary which operates “synchronized” with the base station with regard to the frequencies used via its own test receiver. However, the introduction of such terminals and such base stations is associated with a high cost expenditure and the establishment of an additional infrastructure.
However, if the repeater is configured in the sense of the present invention to reconvert the frequency of the frequency-converted information channel in the downlink direction and to carry out the frequency conversion of the at least one information channel into the frequency from the unused frequency ranges in the uplink direction, by means of the implemented converting device, no cognitive terminal for information exchange is necessary. Instead, the repeater handles the transformation of a cognitive communication network for the terminal in this case. The terminal can then be, for example, a MIMO or multi-frequency terminal which has two or more antennas or operates on two or more mobile radio bands, respectively, which corresponds to the current state of the art.
In the aforementioned variant of the embodiment, the cognitive base station is enabled, in particular, for example on the request of the repeater, to code the communication signals sent out on multi-frequency information channels in accordance with the MIMO method. In this case, the repeater with high transmission capacity on multi-frequency information channels receives a MIMO-coded communication signal. This MIMO-coded communication signal is then reconverted with regard to the frequencies for a MIMO-capable terminal and radiated as MIMO signal having at least two locally multiple equal-frequency information channels on at least two or more antennas. In this case, the invention utilizes, for example in the outdoor area, a multi-frequency cognitive data transmission in order to use the MIMO technology with high transmission capacity for the terminal, particularly in an indoor area.
In a further variant of the embodiment, between the base station and terminal, at least two repeaters are comprised which are configured for receiving, passing through and retransmitting communication signals on multiple information channels and which in each case contain a converting device, connected to the test receiver, for frequency conversion of at least one information channel passed through in at least one communication direction to a frequency from the unused frequency ranges, the converting device of the repeater on the base station side being configured for carrying out the frequency conversion of the at least one information channel to the frequency from the unused frequency ranges in the downlink direction and reconverting, particularly resetting, the frequency of the frequency-converted information channel in the uplink direction, and the converting device of the repeater on the terminal side being configured for reconverting, particularly resetting, the frequency of the frequency-converted information channel in the downlink direction, and carrying out the frequency conversion of the at least one information channel to the frequency from the unused frequency ranges in the uplink direction.
In other words, two repeaters which communicate wirelessly with one another via a respective air interface are provided in this advantageous variant of the embodiment. Cognitive information transmission is used for this wireless communication. The repeater on the base station side receives the multiple information channels of the base station, cognitively converts these information channels to the free frequencies and transmits them wirelessly.
The repeater on the terminal side receives the cognitively converted multi-frequency information channels wirelessly, correspondingly resets the frequencies of the information channels and transmits the signals, for example as multiple information channels, on the assigned communication band of the base station. The repeaters exchange their functionality for the uplink direction, i.e. of a communication of the terminal in the direction of the base station. Here, as also in the variants of the embodiment described previously, it is not necessary for the inventions that the repeater or the converting device, respectively, returns the cognitively converted frequencies into the original frequencies. It is also possible to convert the cognitively frequency-offset information channels into other frequencies than those of the communication band of the base station.
As mentioned, the base station is suitably a MIMO-capable base station and is configured for transmitting and receiving a communication signal on spatially multiple information channels, the repeater on the base station side being configured as a MIMO-capable repeater and thus being capable of transmitting spatially multiple information channels in the uplink direction and receiving spatially multiple information channels in the downlink direction. In other words, the repeater on the base station side communicates with the base station in the MIMO technology. On the terminal side, the repeater communicates on the information channels cognitively offset in frequency. A second repeater communicates with the first repeater on the information channels cognitively offset in frequency. On the terminal side, this second repeater communicates with the terminal in accordance with MIMO technology.
In the aforementioned variant of the embodiment, the repeater on the base station side is suitably connected to the base station, particularly to the antenna feed lines, via bidirectional couplers, the communication signals being exchanged optically or electrically with the base station. In this case, the communication signals coded, for example, for MIMO are exchanged in wire-connected manner with the base station.
The terminal is suitably also a MIMO-capable terminal which is configured for transmitting and receiving a communication signal on spatially multiple information channels, the at least one or terminal-side repeater being a MIMO-capable repeater which is configured for transmitting spatially multiple information channels in the downlink direction and receiving spatially multiple information channels in the uplink direction.
Also preferred, a test receiver is allocated to the or each converting device, the test receivers being connected to one another and “synchronized” with regard to the frequency to be selected for the frequency conversion. This results in a cognitive communication network, the respective converting devices and repeaters exchanging the respective selected frequencies from the free frequency ranges with one another via the test receivers so that the necessary information for frequency conversion and resetting the frequency, respectively, is present in each case.
In another preferred embodiment, the communication network comprises a multiband transmission system with a master unit and at least one remote unit connected to the master unit via a common signal line, the master unit being constructed for transmitting and for receiving the communication signals on multiple information channels and for connecting or decoupling the multiple information channels to a or from a transmission signal, the transmission signal being conducted via the common signal line, and the at least one or terminal-side repeater being constructed as a component of the remote unit. Such a multiband transmission system can be found, e.g., in DE 10 2009 052 936 A1.
In this variant of the embodiment, communication signals are exchanged with the base station, for example via an air interface of the master unit. The multiple information channels are individually frequency-converted with other information channels such as, e.g., from a digital network or the like, if necessary for avoiding interferences, and exchanged between the master unit and the remote units via a common signal line. In this arrangement, the common signal line is, for example, constructed to be electrical or optical so that losses due to spark transmission are avoided. At the remote units, the various multiple information channels are reset in their frequency and transmitted wirelessly. In one variant of the embodiment, the multiband transmission system transmits in particular the information channels cognitively converted in frequency, the transformation of the cognitive frequency-offset information channels taking place in the remote unit. Synchronization with regard to the cognitive frequency conversion can take place, in particular, via the converting device integrated in the remote unit, which can also be connected to a digital network via the common signal line.
For a cognitive network to work, as simple as possible a coordination with other radio services which, in particular, are released for primary users must be aimed for. To eliminate disturbances in the frequency range of these primary users, a central controller is used alternatively or additionally to a metrological checking of the availability (“listen before talk”) of potentially free, that is to say unused, frequency bands in a preferred embodiment. In particular, the cognitive interaction of one or more repeaters with one or more base stations is suitably coordinated in the network.
For this purpose, the or each test receiver of the communication network specified is connected in an advantageous embodiment of the communication network to a central database in which the frequency ranges unused for wireless communication are stored. For this purpose, the test receivers are advantageously equipped in each case with a modem which are connected wirelessly or cable-connected to one another and/or to this central cognitive database. A wireless modem is, for example, a GSM, UMTS or LTE modem. A cable-connected modem can be, for example, an ISDN, DSL or LAN modem. From this database, the test receivers can call up the locally or temporarily free unused frequency ranges and thus carry out the respective cognitive frequency conversion of the information channels by means of the respective converting devices. Conversely, the respective test receivers of the communication network can also deposit the selected frequencies for the other test receivers in the central database where they are made available retrievably.
In an embodiment also suitable for this purpose, the or each test receiver is equipped with a spectrum analyser. This can detect, for example, the locally or temporarily free unused frequency ranges and deposit the information obtained in the central database. The individual test receivers of the communication network thus virtually represent sensors of a cognitive wireless network.
It is especially when the communication network is used for transmission in a mobile umbrella cell that the metrologically supported coordination of the spectrum utilization has a special significance since in this case supply regions of different primary user services may be passed. If GPS data are additionally used or also recorded by the spectrum analyser, the database can also advantageously be fed with spatially resolved information about the spectrum utilization.
The or each converting device is advantageously configured for carrying out no frequency conversion for at least one of the information channels. This makes it possible to save computing capacity and equipment expenditure.
To avoid any feedback of the transmitted communication signal with the communication signal passing through, the or each repeater is optionally equipped with electronic means for suppressing the feedback. The means for suppressing a feedback suitably comprise a digital adaptive filter. For this purpose, a part of the transmitted communication signal is fed back to the adaptive filter and thus its transfer function is changed. At the digital level, this suppresses feedback components in the communication signal passing through.